1. Field of the Invention
The invention relates to hydrophones and in particular to devices of this type which deliver an electric signal in response to the vibratory velocity of the incident acoustic waves, this response being flat over an extended frequency range.
2. Description of the Prior Art
To construct a velocity hydrophone, it is known to use acoustic pressure transducers adapted so as to supply an electric signal characteristic of the pressure gradient within the acoustic wave. Pressure gradient hydrophones are then formed by a pair of cells sensing the pressure at two distinct locations. However, because of the fixed spacing between the cells, the sensitivity varies as a function of the frequency. The velocity hydrophone to which the present invention applies comprises a mobile element plunged in fluid, so as to take on the particular movement generated by the acoustic wave at a given location. Reference is thus made to the alternate bending deformation undergone by the mobile element embedded by its end in a reference mass for developing an electric current by piezoelectric effect. This current forms advantageously the response signal independent of the frequency in a range situated above the natural resonance frequency of the deformable assembly comprising the reference mass.
Thus, in this velocity hydrophone, the electric acoustic transducer element has a lamellar or blade like shape with sufficient flexibility to deliver an electric signal substantially proportional to the particular velocity of the fluid at the level of the wave front received by the hydrophone. In the immediate vicinity of the transducer element, the particular movement of the fluid is complex particularly because the transducer element vibrates under flexion, with a range of movement related to the distance which separates it from the inertial mass in which the transducer element is embedded.
To obtain a response sensitive to the flexion, the transducer element comprises several suitably biased layers.
So that the electric signal delivered is representative of the particular velocity over an extended frequency range, it is necessary to connect the output electrodes of the active piezoelectric element to a user circuit having a low electric impedance with respect to the capacitive reactances of the transducer element.
To improve the response at low frequencies of a speed hydrophone, its resonance frequency should be reduced, contrary to what happens with the pressure hydrophones, where efforts are made rather to extend the response towards the high frequencies by adopting a more rigid structure or a structure with reduced mass.
In the case of the hydrophone of the invention, a choice may be made between materials with low piezoelectric coefficients and low modulus of elasticity such as piezoelectric polymers or materials with high piezoelectric coefficients and high modulus of elasticity such as piezoelectric ceramics. The stiffness and sensitivity depend on the choice of the thickness of the materials used, but the extent and the particular shape of the deformable element are also important, for they condition the extent of the frequency range where a flat response may be reckoned on.